Among practical lasers in widespread use, gas lasers offer some considerable advantages over the solid semiconductor and crystal lasers. The most representative species of gas laser, the helium-neon laser, is easily the most commonly used laser of any type. In fact helium-neon lasers comprise well over half of all lasers in use today. One of the principal reasons for this is certainly the ability of the helium-neon medium to operate stably and reliably at 0.6328 microns, in the visible light range. However, low initial cost is also an important factor in the common use of this laser type.
The operation of gas lasers such as the helium-neon type depends upon the establishment of a glow discharge within a confined column of the gaseous mixture between a pair of electrodes under the influence of a powerful electric field produced by the application of a large potential difference between the electrodes.
The resulting plasma discharge produces a population inversion in which one or more upper energy levels of a gaseous component is artificially overpopulated. The consequent transitions from such an upper energy level to one or more lower levels release energy of discrete wavelengths associated with the transitions concerned.
By providing an optically resonant cavity in the form of a pair of mirrors, one at each end of the glow discharge column, the establishment of reliable and steady operation at a desired one of the possible wavelengths attainable from the lasing medium is possible.
However, in order to provide that the laser be stable in operation, relatively resistant to thermal and mechanical shock, and capable of being started without the excessive application of voltage, a number of important problems must be dealt with in practice.
In particular, the optical axis of the mirror pair must be well aligned with the central axis of the discharge-confining tube or capillary, or else diffraction loss will become unacceptably large, and the performance of the laser will be degraded.
While techniques have been evolved for securing such accurate alignment of the mirrors during final assembly of the laser, too little has been done in the prior art to ensure that the mirrors continue in perfect alignment despite local heating and cooling of the glass envelope of the laser. Such local heating or cooling of portions of the laser envelope result frequently from the mounting of the laser, together with other sources of radiated and convected heat, inside an equipment cabinet, in sunlight, or exposed to another source of heat which tends to cause unequal temperatures to exist in different parts of the laser envelope.
Since glass is in many ways an optimum material to use in the fabrication of the envelope of the laser, it is unfortunate that it lacks the ability to conduct heat sufficiently to ensure that all parts of the envelope operate at the same temperature. Consequently, variations in ambient temperaure over the surface of the envelope are all too often reflected by significant envelope temperature gradients in a direction circumferentially around the tube envelope.
The effect of such gradients is to cause a very slight "sausaging" (bowing or sagging) of the envelope such that the initially near-perfect mirror alignment can easily be lost. One very important result of the misalignment is that "pointing stability", a term used to describe the extend to which the laser is free of angular deviation of the axis of the light beam over a period of time, is substantially degraded. Moreover, there can be sufficient diffraction loss in the optical cavity to significantly reduce power output or, in extreme cases, to prevent successful sustained oscillation such that the laser operation becomes erratic or ceases altogether.
A second problem encountered in practical forms of prior art lasers has been the inadequate support provided to the end of the discharge-confining or capillary tube. Since the bore through this capillary tube may be as little as one millimeter or less while its length may be on the order of 170 millimeters, or more; it becomes obvious that very little flexure of the capillary tube can be tolerated without causing significant misalignment and non-linearity of its axis.
Some prior art lasers of the general type under consideration (see for example U.S. Pat. No. 3,988,698) have supported the "free" end of the capillary (the end not joined to the outer envelope wall) only by means of a rather imprecise and inadequate support, which permitted an unacceptable amount of capillary tube movement in response to shock or vibration. Furthermore, the support was often positioned intermediate the ends of the capillary leaving a cantilevered, unsupported end section, thus compounding the problem.
Other prior art gas lasers have simply provided no capillary tube support inside the laser envelope, such that the entire capillary length was supported as a cantilever from the end joined to the outer envelope wall.
Yet another problem which has affected the operation of gas discharge lasers has been difficulty in initiating the discharge. The reliable initiation of such a glow discharge without resort to excessive starting voltage levels depends upon the existence of a certain residual level of ionization in the gaseous medium. When the gaseous medium is exposed to ambient levels of daylight, photo-ionization is sufficient in most instances. In the absence of visible light (as in the case of enclosure of the glass envelope with an opaque metallic sleeve) sufficient ionization for easy starting is usually produced by a radiation background of low level cosmic rays. Under most conditions of utilization this background radiation is sufficient to provide an adequate residual ionization level in the gases such that glow discharge can be initiated reliably and fairly easily without resort to high voltage starting pulses.
However, when laser operation is attempted under conditions where substantial shielding from both visible and cosmic radiation is present, as it would be during operation in underground locations, then starting becomes unreliable, often taking several seconds to several minutes or more after the power supply is turned on.
Although there are several approaches possible to cure this problem, each has some element of undesirability. For example, while it is possible to incorporate an auxiliary light source within the opaque envelope containing the laser, such an addition is undesirably expensive in the case of low cost lasers typically used for optical instrumentation, surveying, etc. Moreover, the auxiliary light source would consume power, introduce undesirable heat, and add an additional element of possible unreliability.
As has already been noted, reliable starting can for the most part be restored even in locations of extremely low background radiation by sufficiently elevating the starting voltage available from the power supply. However, this approach is severely limited by the necessity of providing adequate electrical insulation, especially of the power supply leads and connections extending from the laser to its power supply. Consequently, this approach too is clearly undesirable.